A torque exchange between clutches of a conventional automatic vehicle transmission is closely controlled via a control module, e.g., a transmission control module (TCM). The control module of such a transmission commands offload of torque capacity of the particular clutch that is associated with a current speed ratio, i.e., the offgoing clutch, and simultaneously applies another clutch associated with a desired new speed ratio, i.e., the oncoming clutch. Torque from one or more sources, typically an internal combustion engine and/or one or more electric traction motors, is then exchanged from the offgoing clutch to the oncoming clutch in order to complete the shift.
The clutches of a transmission may be described in terms of the mode that is used to establish their control. Thus, the offgoing and oncoming clutches may be referred to as “synchronous clutches” in a typical synchronous shift. In an oncoming synchronous clutch, clutch pressure remains fully exhausted while the clutch is still slipping. Clutch pressure is applied only after the synchronous speed is attained. By way of contrast, for an offgoing asynchronous clutch, some amount of clutch pressure is applied to the clutch assembly even after the clutch slips. As a result, an asynchronous clutch is able to produce output torque while slipping.
A hybrid transmission lacks a fixed speed ratio. In other words offloading/oncoming of the clutches of a hybrid transmission are generally not required because of the speed ratio. Also, in a hybrid transmission one may transition from a gear state to a mode in which there is one offgoing clutch. Thus, not all shifts in a hybrid transmission have an offgoing-oncoming clutch combination.